1. Field of the Invention
Embodiments of the present invention generally relate to seismic data processing, and more particularly to a method for computing a pressure signal gradient.
2. Description of the Related Art
Seismic surveying is a method for determining the structure of subterranean formations in the earth. Seismic surveying typically utilizes seismic energy sources which generate seismic waves and seismic receivers which detect seismic waves. The seismic waves propagate into the formations in the earth, where a portion of the waves reflects from interfaces between subterranean formations. The amplitude and polarity of the reflected waves are determined by the differences in acoustic impedance between the rock layers comprising the subterranean formations. The acoustic impedance of a rock layer is the product of the acoustic propagation velocity within the layer and the density of the layer. The seismic receivers detect the reflected seismic waves and convert the reflected waves into representative electrical signals. The signals are typically transmitted by electrical, optical, radio or other means to devices which record the signals. Through analysis of the recorded signals (or traces), the shape, position and composition of the subterranean formations can be determined.
Marine seismic surveying is a method for determining the structure of subterranean formations underlying bodies of water. Marine seismic surveying typically utilizes seismic energy sources and seismic receivers located in the water which are either towed behind a vessel or positioned on the water bottom from a vessel. The energy source is typically an explosive device or compressed air system which generates seismic energy, which then propagates as seismic waves through the body of water and into the earth formations below the bottom of the water. As the seismic waves strike interfaces between subterranean formations, a portion of the seismic waves reflects back through the earth and water to the seismic receivers, to be detected, transmitted, and recorded. The seismic receivers typically used in marine seismic surveying are pressure sensors, such as hydrophones. Additionally, though, motion sensors, such as accelerometers may be used. Both the sources and receivers may be strategically repositioned to cover the survey area.
Streamers of seismic receivers are often used in marine seismic surveying. The streamers typically contain hydrophones for recording pressure fluctuations caused by the seismic waves propagating in the water. Recently, it has been realized that the value of seismic data would be substantially enhanced if the particle motion vector of the seismic waves propagating in the water could be recorded. Some seismic streamers have included geophones or accelerometers for recording particle velocity or acceleration; however, such configurations have proven to be difficult due to noise and mechanical vibrations in the streamers.
However, according to Newton's Equation of Motion, particle acceleration is equivalent to pressure gradient. As such, pressure gradient may be recorded or estimated, as opposed to particle velocity or acceleration. Pressure gradient may be recorded or estimated using closely spaced multiple streamers, such as over/under configurations. Unfortunately, such methods are often operationally complex and costly. Consequently, it has recently been proposed to use multiple hydrophones spaced closely apart within one streamer. One of the challenges encountered in using such streamers involves keeping the diameter of the streamers to a minimal to avoid negative consequences, such as drag and the like. Consequently, this requirement limits the amount of distance that can be placed between the receivers on the streamers in a pressure gradient configuration. It has been observed that the shorter the distance between the receivers, the lesser the amount of low frequencies that can be recovered.
One of the problems of placing the receivers in close proximity to each other is the increase in noise behind the signal in pressure gradient estimate. As the distance between the receivers decreases, the signal strength of the difference between the pressure signals of the receivers also decreases. (See FIG. 1). This decrease in signal strength often leads to unreliable pressure gradient measurements.
Therefore, a need exists in the art for an improved method for computing the pressure signal gradient that will overcome the above referenced problems.